Varying effects of greenness in the spring and summer on the development of allergic rhinitis up to 27 years of age: The Espoo Cohort Study
Authors: Inês Paciência, Aino K. Rantala, Harri Antikainen, Timo T. Hugg, Maritta S. Jaakkola, Jouni J. K. Jaakkola
Journal: Allergy (2023)

The world has experienced considerable growth of urban areas in recent decades, with approximately 54% of the world population living in cities.1 While this development provides some benefits, unplanned, uncontrolled and rapid urbanization has also been associated with environmental degradation, land changes and loss of green areas.2 Epidemiological studies on the association between urban greenspaces and development of allergic diseases have provided inconsistent results.3 This heterogeneity in the effects of greenspaces on the development of allergic rhinitis could be explained by the season of exposure. Thus, we aimed to estimate potential associations between exposure to greenspaces during pregnancy and early life, in spring and summer seasons, and development of allergic rhinitis up to 27 years of age. The study comprised data collected in the Espoo Cohort Study, which included 2568 children delivered between 1984 and 1990 who were living in 1990 in the city of Espoo in Southern Finland, and who had been followed regularly since the birth (Appendix S1).4 The outcome was the development of allergic rhinitis during the follow-up time. Greenspace was assessed using the mean Normalized Difference Vegetation Index (NDVI) within 300 m of the participant's residence during pregnancy and the first 2 years of life during spring and summer periods (Table 1, Figure S1). We applied Cox proportional hazards regression to estimate adjusted hazard ratios (aHRs) and 95% confidence intervals (95% CI) for the associations between the cumulative exposure to NDVI (per 1 unit-month increase), and the risk of allergic rhinitis, measured as the time of onset of allergic rhinitis up to 6, 12 and 27 years of age. The Ethics Committee of the Oulu University Hospital (Oulu, Finland) approved the study, and informed consent was obtained from all participants or their caregivers. Figure 1 shows the adjusted associations between residential exposure to NDVI at specific time points and allergic rhinitis. There was no clear evidence of association between the cumulative exposure to NDVI during pregnancy and allergic rhinitis. However, an increase in exposure to NDVI during spring season in early life was associated with an increased risk of allergic rhinitis up to 12 years of age [aHR (95% CI) = 1.726 (1.078; 2.765)] and up to 27 years of age [aHR (95% CI) = 1.703 (1.139; 2.545)]. During the summer, an increase in the exposure to NDVI was associated with a decrease in the risk of allergic rhinitis up to 12 years of age [aHR (95% CI) = 0.754 (0.585; 0.972)] and up to 27 years of age [aHR (95% CI) = 0.801 (0.649; 0.989)]. Our findings provide new evidence that early-life exposure to greenspaces during the spring increases the risk of developing allergic rhinitis up to young adulthood. Conversely, during the summer, exposure to greenspaces reduces the risk of allergic rhinitis. During the summer, the highest NDVI levels, which indicate a higher density of vegetation, may reduce the levels of air pollutants,5 as well as create a cooling effect through shade and evapotranspiration, and thus generate airflows and disperse the concentrations of pollutants.6 However, during the spring, the greenness is linked to the highest levels of birch tree pollen, the most common cause of pollen sensitization rate in Finland [prevalence 34.0% (26.8–41.2%)].7-9 Furthermore, the higher levels of air pollutants in Espoo during the study period10 may have induced changes into the chemical composition of pollens, intensifying their allergenic potential and thus, increasing allergen-induced inflammatory and cellular immune responses,3 which consequently increase the risk of allergic rhinitis. The important bi-directional role of the season in the effects of exposure to greenspaces on allergic rhinitis is likely to be explained by qualitative and quantitative changes in vegetation over the seasons. Thus, the provision of diverse greenspaces in the residential neighbourhoods during early life may be an effective strategy to prevent allergic diseases. Inês Paciência contributed to the conceptualization, methodology, formal analysis, investigation, writing—original draft, writing—review and editing. Aino K. Rantala contributed to the conceptualization, methodology, formal analysis, and writing—review and editing. Harri Antikainen contributed to the methodology, and writing—review and editing. Timo T. Hugg contributed to the conceptualization, methodology, and writing—review and editing. Maritta S. Jaakkola contributed to the conceptualization and writing—review and editing. Jouni J. K. Jaakkola contributed to the conceptualization, methodology, writing—review and editing, and supervision. The authors gratefully acknowledge the University of Oulu and the Academy of Finland Profi Biodiverse Anthropocenes no. 336449 and Academy of Finland grant no. 310371 and no. 310372 (GLORIA consortium). This study was funded by the Academy of Finland (no. 310372 and no. 336449). The authors declare no conflicts of interest. Appendix S1. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.